"unroutable domain" mail issues

Ok so hundreds of emails are bouncing from one of our servers every day, and its driving me mad.
The only domains that seem to bounce are @hotmai.com @yahoo.coim and @gmail.com
It makes no diffrence which domain i send the mail from.
The error only occures if its sent via a PHP script, if its sent by outlook for example there will be no errors.

I need a soultion to this ASAP as clients are starting to moan alot about this, and it is kind of anoying for my billing system.

PLEASE HELP

 

 

 

 

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